Hafnium base alloy (boron)

ABSTRACT

Hafnium tantalum alloy containing about 15 and about 35 weight percent tantalum, between about 0.03 and about 2.0 weight percent boron, and at least one additional alloying agent selected from chromium, silicon and aluminum.

United States Patent Vernon L. Hill Niles;

Harry R. Nichols, Chicago, both of III. 769,397

Oct. 21, 1968 Nov. 23, 1971 HT Research institute Chicago, Ill.

[ 72] inventors 2i Appl. No. [22] Filed [45 Patented [73] Assignee [54] HAFNIUM BASE ALLOY (BORON) 51 mu ..C22c27/00 Primary Examiner-L. Dewayne Rutledge Assistant ExaminerE. L. Weise Altorney- Fitch, Even, Tabin & Lucdeka ABSTRACT: Hafnium tantalum alloy containing about l5 and about 35 weight percent tantalum, between about 0.03 and about 2.0 weight percent boron, and at least one additional alloying agent selected from chromium, silicon and aluminum.

HAFNIUM BASE ALLOY (BORON) This invention relates generally to hafnium base alloys, and more particularly it relates to hafnium-tantalum alloys having improved oxidation and corrosion resistance at elevated temperatures.

There has existed for some time a need for structural metals and alloys which have good corrosion and oxidation resistance at elevated temperatures, e.g., above 2200 F. In addition to corrosion resistance, a desirable metal should have good structural strength at temperatures above 2200 F. and should be able to be fabricated and worked both before and after exposure to high temperature oxidation. The corrosion resistant metal should also be capable of being welded without embn'ttlement.

Hafnium is a ductile metal which has a melting point above 4000 F. The ductibility of hafnium is such that it does not possess good structural strength at elevated temperatures. The addition of between about to about 60 weight percent tantalum to hafnium provides a hafnium-tantalum alloy which has sufiicient strength at elevated temperatures to be useful in the manufacture of structural parts. Hafnium-tantalum alloys are also readily fabricated by hot working at l800 F. to 2500 F., and when the alloys contain relatively large amounts of tantalum, the alloys may be made cold workable by heat treatment and/or by the addition of minor amounts of molyb denum.

l-lafnium oxidizes more slowly than does tantalum at temperatures above 2000" F. to form a stable dioxide, HfO However, the hafnium-tantalum oxide surface scale layer which forms on the surface of hafnium-tantalum alloys upon oxidation at high temperatures affords limited protection against oxidation, principally because of the high oxygen mobility of hafnium-tantalum oxide and the spalling of the hafnium-tantalum oxide surface scale layer due to the mismatch in the expansion coefficient of the hafnium-tantalum oxide scale as compared to the expansion rate of metallic hafnium-tantalum alloy substrate. Hafnium-tantalum oxide is also quite porous which permits diffusion of oxygen through the oxide layer to the metallic surface, resulting in further undesirable oxidation.

Hafnium-tantalum alloys containing between about 15 and about 35 weight percent tantalum have been found to have desired structural strength and fabricability both before and after high temperature oxidation. However, exposure of hafnium-tantalum alloys to oxidative corrosion at temperatures above 2000 F. results in extensive spalling of the protective oxide layer which forms on the surface of the alloy. This spalling is sufficiently severe that failure occurs after high temperature oxidation exposure of 30 hours or less. Generally, the best corrosion resistance of the binary hafnium-tantalum alloys is obtained when the alloy contains between about and about weight percent tantalum. However, binary hafniumtantalum alloys containing 20 to 25 weight percent tantalum do not exhibit useful lives greater than about hours exposure to oxidation at 2500 F.

It is a principal object of the invention to provide improved hafnium base alloys. A further object is to provide hafniumtantalum alloys having improved high temperature resistance to oxidation. A further object is to provide hafnium-tantalum alloys of good structural strength and fabricability which are able to withstand extended exposure to high temperature oxidative environments. Another object is to provide hafniumtantalum alloys which are able to withstand cyclic temperature environments without excessive oxidation. Still another object is to provide hafnium-tantalum alloys which exhibit improved resistance to oxidation at high temperatures, and also exhibit resistance to oxidation at lower temperatures.

These and other objects of the invention will be more readily understood from the following detailed description.

Generally, the invention is directed to a hafnium base alloy which includes between about 15 and about weight percent tantalum, between about 0.03 and about 2.0 weight percent boron, and at least one additional alloying agent selected from chromium, silicon and aluminum, the balance being hafnium. The additional alloying agents may be present in the hafniumtantalum-boron alloy in differing amounts, depending on the particular additional alloying agent, and the presence of other of the additional alloying agents. Chromium may be present in an amount between about 0.5 and about 5.0 weight percent. The addition of substantially greater than about 5.0 weight percent chromium tends to cause spalling of the oxide scale, and for most purposes it is generally desirable to maintain the chromium content below about 5.0 weight percent. Silicon may be present in an amount between about 0.2 and about 3.5 weight percent, and aluminum may be present in an amount between about 0.3 and about 3.0 weight percent.

When chromium is present as an additional alloying agent it is generally preferable to include between about 0.2 and about 2.0 weight percent boron in order to provide maximum high temperature corrosion resistance. When the additional alloying agent is silicon or aluminum and in the absence of chromium, the boron content may be reduced to as low as 0.03 weight percent. When the alloy contains both chromium and aluminum as additional alloying agents, the aluminum content may be reduced to as little as about 0.05 weight percent.

As indicated, the base metal of the improved alloy composition is hafnium. Generally, the hafnium should be of high purity, but minor amounts of those impurities normally associated with hafnium, such as zirconium can be tolerated. It is to be understood that use of the tenn hafnium" is intended to include those impurities normally associated with commercial grades of hafnium.

The alloy may include between about 15 percent and about 35 weight percent tantalum, which may be any commercial grade of tantalum along with the impurities normally associated therewith. It has been determined that if the tantalum content exceeds about 35 weight percent, the high temperature oxidation resistance of the resulting alloy is reduced. Below about l5 weight percent tantalum, the resulting hafnium-tantalum alloy does not have desired structural strength or oxidation resistance. Generally, it has been determined that alloys containing between about 20 and about 30 weight percent tantalum provide the best combination of structural strength and resistance to high temperature oxidation.

As indicated, the surface oxide scale layer which is formed on binary hafnium-tantalum alloys when exposed to oxidative environments at high temperatures, e.g., 2500 F is relatively porous and friable, which permits excessive oxygen mobility through the oxide scale to the metal substrate. Further, the expansion mismatch between the oxide scale layer and the alloy substrate results in extensive spalling upon extended exposure to oxidation at high temperatures.

lt has been determined that the addition of specific alloying agents to hafnium-tantalum alloys substantially improves the ability of the hafnium-tantalum alloy to withstand extended exposure to high temperature oxidation. The addition of the alloying agents decreases the porosity and modifies the structure of the oxide scale layer formed during high temperature oxidation. This lessens diffusion of oxygen to the alloy substrate surface and decreases the rate of corrosion. In addition, it has been found that the addition of certain alloying agents modifies the metal plus oxide subscale layer which exists between the outer fully oxidized surface scale layer and the alloy substrate.

Hafniurn-tantalum alloys of improved resistance to high temperature oxidation may be obtained through the addition of minor amounts of boron and at least one of chromium, silicon or aluminum. The boron content of the alloy is desirably between about 0.03 and about 2.0 weight percent. The addition of boron as the sole alloying agent, e.g., hafnium-tantalum-boron ternary alloys, does not provide sufficient oxidation resistance for most purposes, and, in accordance with the invention, the alloy contains at least one additional alloying agent selected from chromium, silicon and aluminum in addition to boron.

The boron content of the alloy is desirably between about 0.2 and about 2.0 weight percent when the additional alloying agent is chromium. When the alloy contains silicon or aluminum as the additional alloying agent the boron content may be reduced, for example to 0.03 weight percent boron without detrimentally afifecting the high temperature corrosion resistance of the hafnium-tantalum alloy.

Resistance to high temperature oxidative corrosion is determined by exposure of gram are melted buttons of the various alloys to high temperature oxidation for varying periods of time. In all of the tests set forth herein the alloy buttons were annealed at 2500 F. for 2 hours prior to oxidation. During the oxidation tests the buttons were supported in high purity alumina boats in the ambient atmosphere. The samples were considered to fail when one or more of the following effects were observed: (1) excessive spalling, (2) extensive cracking of the surface oxide scale layer, or (3) high weight gain.

Generally, cracking of the surface oxide scale is not com sidered to be a failure unless accompanied by one of the other effects. However, in most tests if cracking of the oxide scale occurred, the test was terminated after the next exposure cycle. The oxidation rate of each of the alloy buttons was measured in terms of weight increase, as compared to original weight. Measurement of percent weight gain is an accurate method of comparing the oxidation resistance of alloys of similar composition and size, but should not be used to quantitatively compare the oxidation resistance of alloys containing widely varying alloying agents due to the differences in rate of oxidation and affinity for oxygen of the different alloying agents.

There is illustrated in table II the results of oxidation studies in ambient static air at 2500 of hafnium-tantalum alloys containing boron as an alloying agent, and two additional alloying agents selected from chromium silicon and aluminum.

The significant improvement in oxidation resistance of hafnium-tantalum alloys containing boron and at least one additional alloying agent is further illustrated by extended oxidation studies at 2500 F. for up to 500 hours. The oxidation studies were carried out in accordance with the test procedures outlined herein except that the tests were extended from l00 hours to 500 hours. The results of these studies, using samples similar to those of tables I and II confirm that hafnium-tantalum alloys containing boron and at least one additional alloying agent are capable of withstanding 500 hours exposure to oxidation in static air at 2500 F. without failure. This represents a significant improvement over previously known structural alloys.

It can readily be seen that hafnium-tantalum alloys containing boron and at least one additional alloying agent have good oxidation resistance at temperatures above 2200 F. However, such alloys which do not contain aluminum exhibit rapid oxidation at lower temperatures, e.g., l200 F. to l800 F. It is believed that the susceptibility of the alloys to low temperature oxidation is due primarily to the inability of the outer oxide scale layer to be self healing at lower temperature. That is, the cracks which form in the oxide layer are not effectively sealed at low temperatures. It is also possible that at lower STATIC AIR AT 2,500 F.

Percent weight gain after exposure in hours Alloy composition, weight per- Example cent (atomic percent) 1 5 22 64 80 100 1 Ta-0.13, 13-0-32, Si (23) 0. 62 1.4 2. 57 3.32 4.0 4. 4 4. 87 2 Ta-0.13, B-0.8, Si (2B) 0. 51 0.99 1 2.08 2. 8 3. 5 3.9 4.3 3 HI-24 .3, Tfl-O-GG, B-0.8, S1 (1013) 0.49 1.83 2.39 3.04 3.39 3. 77 4 Til-0.33, B09, A1 (513) 0.48 0. 99 2. 54 3. 54 4. 6 5. 2 1 5. 5 Hli/hffl-Ofifi, 3-1.3, A1 (108) 0.48 1.0 2.3 3.14 4.0 4. 64 5:3 6 Iii-52x12, Tia-0.66, B-0.9, Al (10B) 0.47 0.91 2.16 2.93 3.77 4.33 4.96 7 Hf(-52.rl,.Ta-0.66, B-1.5, Cr (10B) 0.66 1.23 2.28 2.94 3.58 3.95 4.3 8 Ht-52.r5),-Ta-0.655, B-L5, Or (1013) 0.66 1.23 2.28 2.94 3.58 3.95 4.3

1 Scale cracked.

There is set forth in table I the results of oxidation studies in static ambient air at 2500 F. of hafnium-tantalum alloys containing boron and one additional alloying agent selected from chromium, silicon and aluminum. Each of the examples of table I illustrate the substantially improved results and superior resistance to oxidation at elevated temperature which is obtained when the hafnium-tantalum alloy contains the disclosed alloying agents. Without the addition of such alloying agents, the hafnium-tantalum alloys fail to survive more than 30 hours exposure to high temperature oxidation.

temperatures the subscale layer is not sufficiently formed or is 0 absent so that the expansion mismatch between the substrate and the oxide layer is not buffered.

Oxidation studies at temperatures between l200 F. and 1800 F. have shown that the presence of aluminum as the sole alloying agent in a hafnium-tantalum alloy provides substantial protection against oxidative corrosion within this temperature range. However, hafnium-tantalun alloys containing aluminum as the sole alloying agent are susceptible to rapid oxidation at temperatures of 2500 F. with the resulting failure of the alloy.

TABLE II.OXIDATION RATE OF HAFNIUM, ALLOYS IN STATIC AIR AT 2,500 F.

Percent weight gain after exposure for- Alloy composition, weight percent (atomic Example percent) 1 hr. 5 hrs. 22 hrs. 40 hrs. 64 hrs. hrs. 100 hrs. 1 Hf-24.5Ta-0.655B-1.5Cr-0.12Al (10B-5Cr-0.5A1). 0.9 1. 75 3.20 4.06 5.4 5.86 2 Hf-24.5Ia-0.655B-1.5Cr-0.%Al (10B-5Cr-1Al) 0.92 1. 81 3. 5 4. 63 3 HI-Z3.5Ta-0.655B-1.5Cr-0.36Al (10B-5Cr-L5Al). O. 8 1. 56 3.26 4. 2 1 6.15 4 Ht'-23.6'Ia-0.655B-1.5Cr-0.72Al (10B-5Cr-3Al). 1. 03 5. 63 5 Hf23.5Ts-0.655B-1.5Cr-0.92A1 (10B-5Cr-5A1)... 1. 12 3. 64 4.07 5.03 6. 13 6 Hf-24.5'Ia-O.79B-l.5Cr-0.12A1 (12B-5Cr-0.5Al) 0. 85 1. 60 2. 89 3. 67 4. 87 5. 26 7. Hf-24.5Ta0.852B-l.5Cr-0.12Al (13B-5Cr-0.5Al) 0. 86 1. 67 2. 99 3. 78 5. 0 5. 35 8. H1-24.5Ta-0.79B-1.5Cr-0.36Al (12B-5Cr-1.5Al) 0. 95 1. 90 3. 75 4. 37 5.35 6. 3 9. Ht-24.5Ta-0.655B-1.2Cr-0.12Al (10B-4Cr-0.5Al) 0.89 1. 7 3. 07 3. 89 5. 12 5. 53 10 H[-24.5Ta-0 655B-1 2Cr-0 36Al (l0B-4Cr-L5Al) 0. 92 1. 69 3. 22 4. 05 5. 5 6. 1 11 Hf-23.5Ta4) 66B-0.9Al-1.5Cr (10B) (5A1) (5Cr). 0.63 2.3 3.0 4.14 4.5 12 Hf-23.3Ia-0.66B-1.3Al-1.5Cr (10B) (8A1) (5Cr) 2 0. 59 1. 17 2. 14 13. Hf-23.5Ia-0.66B-O.8Si-1.5Cr (10B) (581) (500.. 0.54 1.84 2. 3 3. 1 3. 38

1 Scale cracked. 2 Scale spalled.

The low temperature oxidation resistance of hafnium-tantalum alloys containing boron and at least one additional alloying agent may be substantially improved if aluminum is employed as an additional alloying agent. The aluminum provides low temperature resistance to oxidation, and the combination of boron and aluminum, and the other additional alloying agents, provide the high temperature oxidation resistance disclosed herein.

The low temperature oxidation resistance of hafnium-tantalum alloys containing boron and at least one additional alloying agent can also be improved by preoxidation, that is, by exposure of the alloy to elevated temperatures above 2200 F. prior to exposure at lower temperatures. It is believed that preoxidation causes the formation of oxide and subscale layers which aid in preventing corrosion when the alloy is exposed to temperatures of 1200 F. to 1800" F.

Examination of the results of a number of oxidation studies tends to indicate that the degree of spalling that occurs is somewhat dependent upon the relative proportion of chromium and aluminum present in the alloy. When the alloy contains both chromium and aluminum as additional alloying agents it is generally considered preferable to adjust the aluminum and chromium content of the alloy in an inverse relationship, so that a low chromium content higher concentrations of aluminum are employed, and at high chromium content, lower concentrations of aluminum are employed. Good results during low temperature oxidation have been obtained with between about 1.0 and 1.5 weight percent aluminum.

In addition to the static air oxidation corrosion studies, the results of which are set forth in tables I and ll, oxidation studies were also conducted under cyclic conditions. The cyclic oxidation studies were carried out using herein described gram arc melted buttons supported in alumina boats. The samples were heated to 2500 for 1 hour and then removed from the fumace and cooled to room temperature. Upon reaching room temperature the samples were replaced in the furnace and heated to 2500 F. for another hour. Heating to 2500 F. for l hour followed by cooling to room temperature constituted 1 thermal cycle.

The weight gain during thermal cycling oxidation studies is generally comparable to the weight gain during static oxidation at 2500 F. Cracks were observed on the surface of the samples subjected to thermal cycling and these cracks were generally concentrated at the edges and corners of the samples. However, none of the cracks which occurred resulted in accelerated oxidation and microscopic examination showed that the cracks were limited to the surface oxide scale layer and did not propagate to the alloy substrate.

The results of the thermal cycling studies illustrate that the susceptibility of the described hafnium-tantalum alloy to increased oxidation and corrosion at relatively low temperatures, e.g., 1200 to 1800 F. does not occur under conditions of thermal cycling from room temperature to high temperatures, e.g., above 2200 F. Thus, it would be possible to utilize the disclosed alloys in environments which include thermal cycling between room temperature and elevated temperatures without fear of excessive oxidation.

The oxidation resistance of substrates cladded with the described hafnium-tantalum alloys was tested by sandwiching a pure tantalum substrate between various hafniumqantalum alloy cladding materials containing the described alloying agents. The sandwich assemblies were hot rolled air at 2500 F. to a total reduction in area of 65 percent. Excellent bonding of the cladding material to the tantalum substrate was achieved.

Exposure of cladded tantalum substrates to 2500 F. for periods up to hours showed that the hafnium-tantalum alloy cladding containing the described alloying agents sufficiently protected the tantalum substrate which remained soft and ductile. The cladding material was, in some instances, completely converted to oxide scale and subscale, but even in this condition prevented oxidation contamination of the tantalum substrate.

It will be seen that desirable hafnium-tantalum alloy compositions have been disclosed which have substantially improved resistance to oxidation at elevated temperatures. The hafnium-tantalum alloys have good ductility and workability and may be readily fabricated. Further, the hafnium-tantalum alloy compositions are corrosion resistant under both static conditions and cyclic conditions.

Although certain features of the invention have been described with particularity, alternative embodiments within the skill of the art are contemplated.

Various of the features of the invention are set forth in the v following claims:

What is claimed is:

l. A hafnium base alloy having improved resistance to oxidation at high temperatures comprising, between about 15 and about 35 weight percent tantalum, between about 0.03 and about 2.0 weight percent boron, at least one additional alloying agent selected from between about 0.5 and about 5.0 weight percent chromium, between about 0.2 and 3.5 weight percent silicon, and about 0.3 and 3.0 weight percent aluminum, the balance being hafnium.

2. A hafnium base alloy in accordance with claim 1 comprising between about 20 and about 30 weight percent tantalum.

3. A hafnium base alloy in accordance with claim 1 comprising between about 0.2 and about 2.0 weight percent boron and between about 0.5 and about 5.0 weight percent chromium.

4. A hafnium base alloy in accordance with claim 3 comprising between about 0.5 and about 3.0 weight percent aluminum.

5. A hafnium base alloy in accordance with claim 1 comprising between about 0.03 and about 2.0 weight percent boron and one additional alloying agent selected from between about 0.2 and about 3.5 weight percent silicon and between about 0.3 and about 3.0 weight percent aluminum.

6. A hafnium base alloy in accordance with claim 5 wherein the additional alloying agent is silicon.

7. A hafnium base alloy in accordance with claim 5 wherein the additional alloying agent is aluminum.

t I i l UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3 I 522 309 Dated Nonember 2 1 91 Inventor(s) Vernon L. Hill and Harry R. Nichols It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

Column 3, Table I (in the heading) change "2500 F" to Column 4, line 56, change "tantalun" to -tantalum-;

Column 5, line 24, change "a" to at--;

Column 6, line 34, after "and" insert -between-.

Signed and sealed this 9th day of May 1972.

(SEAL) Attest:

EDWARD I'LFLETCHERJR. ROBERT GOTTSCHALK Attesting Officer Commissioner of Patents FORM PO-10 4 1 USCOMM-DC 60376-F'b9 U 5. GOVERNMENT PRINTING OFFICE I969 0-366-33 

2. A hafnium base alloy in accordance with claim 1 comprising between about 20 and about 30 weight percent tantalum.
 3. A hafnium base alloy in accordance with claim 1 comprising between about 0.2 and about 2.0 weight percent boron and between about 0.5 and about 5.0 weight percent chromium.
 4. A hafnium base alloy in accordance with claim 3 comprIsing between about 0.5 and about 3.0 weight percent aluminum.
 5. A hafnium base alloy in accordance with claim 1 comprising between about 0.03 and about 2.0 weight percent boron and one additional alloying agent selected from between about 0.2 and about 3.5 weight percent silicon and between about 0.3 and about 3.0 weight percent aluminum.
 6. A hafnium base alloy in accordance with claim 5 wherein the additional alloying agent is silicon.
 7. A hafnium base alloy in accordance with claim 5 wherein the additional alloying agent is aluminum. 